Metal coating process



as Patented July 2,1946

UNITEDV,STATES PATENT orrics Alfred Douty, Elkins Park, and Gerald G. Romig, assignors to American Chemical Paint Company, Ambler, Pa., a cor- Melrose Park, Pa.,

poration of Delaware No Drawing. Application November 14, 1944, Serial N0. 563,458

4 Claims.

This invention relates to the art of finishing metal with a coat of paint, varnish, enamel, japan or other similar siccative coating. It is particularly concerned with the provision of an improved method of preparing ferrrous metal surfaces for the reception of such siccative coats.

The principal objects of the invention are to improve the adherence of siccative coatings, to increase their life, especially under corrosive influences such as those which are associated with humid atmospheres, salt water, acid fumes, or the like,- and to prevent the appearance of blisters in the siccative coats, especially when these are subjected to humid atmospheres.

We have found that superior results can be obtained in the finishing of metal surfaces, particularly ferrous metal surfaces, by first treating the surface with an aqueous solution containing both primary and secondary phosphates of one or more of the alkali metals and/or of ammonium and then rinsing the surface with a dilute solution containing chromic acid which is allowed to dry upon the work before the siccative coatin is applied.

More specifically, the phosphate mixtures which we employ are such as have a pH higher than that corresponding to the monobasic phosphates present alone and lower than that corresponding to the dibasic phosphates alone. Such solutions contain no free phosphoric acid nor any tertiary phosphates. the surface of grease, oil, dirt and the like and their action in this respect may be augmented, if desired, by the inclusion of organic detergents of various classes and may even include a certain amount of neutral inert salts such as sodium sulfate, for example.

An example of a suitable solution of this kind is the following:

Mono ammonium phosphate oz 1.85 Di sodium phosphate oz 0.15 Organic detergent 1 oz- 0.10 Water g 1.00

The organic detergent of the above formula may be of the class of the well-known organic detergents of good wetting and emulsifying power. As examples of wetting agents suitable for use in the above formula, we may employ sodium alkyl sulfates such as sodium lauryl sulfate sold under the trade name Dupanol; sulfonated hydrocarbons such as alkylated naphthalene sulfonic acids, etc. (commercial products of this kind are sold under the names Santomerse," Naccanol," etc.); and non-ionic detergents which are derivatives of poly ethylene glycol (some of these are sold under the trade names Igepal," "Triton NE, etc.). Certain cation-active materials are likewise suitable. In general, any organic detergent sufficiently soluble and stable at the pH of the solution and of adequate surface tension depressant and emulsifying power may be used.

Solutions of this kind will free The strength of the solution and the proportion of ingredients may be varied widely to suit specific conditoins. In general, solutions within the pH range of 4.7 to 6.5 are preferred as these have the widest general applicability.

After the treatment just described the surface is rinsed in accordance with the following procedure:

Although not always essential a preliminary water rinse may be employed followed by a rinse with a dilute solution containing chromic 'acid. The chromic acid rinsing solution should preferably be used hot and should be allowed to dry upon the work before the siccative coating is applied.

In its preferred form, the chromic acid solution is prepared by dissolving from 1 oz. to 16 oz. of chromic acid per gallons of water. For ordinary commercial purposes, 4 oz. of chromic acid per 100 gallons of water is preferred and while a strength greater than 8 oz. per 100 gallons of water seems to be unnecessary, it appears to do little, if any, harm iintil somewhat over 16 oz. are used. Stronger solutions of chromic acid are likely to injure the subsequently applied siccative coating.

Because of the adherence to the work of some of the alkali phosphate treating solution, an accumulation of these phosphates and of other soluble constituents of the treating solution tends to occur in the chromic acid rinsing solution. Small quantities of such soluble contaminants in the chromic acid rinsing solution can be tolerated without appreciable impairment of the adhesion and life of the siccative coating but larger quantities of these soluble materials, as indeed excessive amounts of chromic acid itself, are definitely harmful. With the foregoing considerations in mind and after extensive experiments we have found that the quality of the chromic acid rinsing solution can be expressed in terms of the following criteria:

a. The solution must contain from 1 to 16 ounces of total chromic acid per 100 gallons.

b. It must not contain more soluble salt of any kind than the metal equivalent of 30 ounces of sodium dihydrogen phosphate per 100 gallons.

c. It must have an acidity corresponding to a pH within the range of 2.0 to 4.6, inclusive.

When traces of sodium phosphate, for example, are carried over into and accumulate in a solution of, say, 8 oz. of CrOa per 100 gallons, the pH, which is initially in the neighborhood of 2.3, begins to rise. The rinsing bath will still be found to produce good results when as much as the equivalent of 30 ounces of sodium dihydrogen phosphate has accumulated; the pH will now be about 2.7. When the equivalent of 50 ounces of NaHzPO; per 100 gallons has accumulated in the rinsing solution, applied siccative coatings will be found to have a notably poorer resistance to salt spray and high humidity.

In the following table we have indicated some of the results obtained in actual tests where panels of ordinary mild steel such as is used in the manufacture of automobile bodies were em,- ployed. All the panels were treated identically in a solution containing mono and disodium phosphate. They were then rinsed in cold tap water following which each panel was immersed for one minutein a boiling chromic acid rinse solution, slowly withdrawn, and allowed to dry. The rinsing solutions were different for each panel as described in the table. All of the panels when dried and cooled were given a coat of a white refrigerator enamel and subjected to accelerated testing in a humidity cabinet at 120 F. and 100% relative humidity. After fortyeight hours the observed results were as follows:

Rinsing solution Humidity cabigfb pH net results per 100 per 100 gals. gals.

0 7.0 Blistered. 0 l0 4. 7 Badly blister-ed. 0 20 4. 65 Do. (i 30 4. 60 Do. 0 50 4. 50 Do. 1 0 3. 51 No blisters 1 i0 3. 44 Do. i 20 3.68 D0. 1 30 3. 88 D0. 1 50 4. 14 Bad blisters 2 0 3. 30 N0 blisters 2 l0 3. 18 D0. 2 20 3. 40 D0. 2 30 3. 48 Do. 2 50 3. 71 Some blisters. 4 0 2. 60 No blisters. 4 l0 2. 70 Do. 4 20 2. as Do. 4 30 3.10 Do. 4 50 3. 28 Some blisters. 8 0 2. 30 N o blisters. 8 l0 2. 40 Do. 8 20 2. 64 Do. 3 30 2. 71 Do. 8 50 2. 90 Some blisters.

Further experiments have shown that even in the absence of metallic salts, an excess of chromic acid, or a pH of the rinsing solution lower than about 2, leads to poorer paint life and to blistering.

Moreover, even when the solution contains an adequate, but not excessive, quantity of chromic acid, rise in thepH of the solution much above 4.6 generally produces poor results.

It is preferable to employ a simple water rinse after the treatment with the alkali phosphate solution and before the use of the chromic acid rinse so as to avoid contamination of the latter but no treatment is necessary after the chromic acid rinse and before the application of the siccative coating. If any subsequent rinsing takes place it should be with distilled water or at least with water which does not contain sufficient impurities to produce blisters in the subsequently applied siccative coating.

The treatment in the chromic acid rinse needs on y to be suiiiciently long to replace thoroughly any previous solution or rinse water so that the work may be thoroughly wetted with the chromic acid solution and, since the work must be dried after the chromic acid rinse and before the application of the finished coat, it is preferable to use the chromic acid solution at a temperature approaching the boiling point so that the metal may be ried more readily. However, temperatures as ow as room temperature or even lower are perfectly satisfactory provided suitable means are available for thoroughly drying the surface after the chromic acid rinse.

In conclusion, "we should like to call attention to the fact that a rinsing solution of chromic acid within the pH range 2.0 to 4.6 such as described above may gradually accumulate a small amountof phosphoric acid as the result of reaction between dragged-over phosphate and the chromic acid of the rinsing solution. However,

we have found that this does no harm so long as the solution's constants remain within the stated limits. For this reason we contemplate including within the scope of the invention the use of rinsing solutions containing both phosphoric and chromic acids whether the phosphoric acid has been deliberately added or has accumulated during the use of the solutions. In some cases, indeed, the deliberate addition of phosphoric acid to the rinsing solution has proven to be actually beneficial so long as the quantity added is such that the solutions pH does not fall below 2.0.

In the following claims the expression alkali phosphates is intended to include ammonium phosphate.

We claim:

1. In the art of finishing ferrous metal surfaces, the method which includes treating the surface with a composition which consists essentially of an aqueous solution of both primary and secondary phosphates from the class which consists of alkali metals and ammonium, the pH of which lies between 4.7 and 6.5 and is greater than that corresponding to the mono-basic phosphate present alone and lower than that corresponding to the dibasic phosphate present alone, rinsing the surface so treated with a dilute aqueous solution containing chromic acid having a pH in the range 2.0 to 4.6. inclusive, and containing no more soluble salt than corresponds to the metal equivalent of 30 oz. of sodium dihydrogen phosphate per gallons, drying the rinsed surface and then applying a siccative finishing coat to the dry surface.

2. The method of claim 1 in which the dilute chromic acid rinsing solution contains from 1 to 16 oz. of chromic acid per 100 gallons of solution.

3. The method of claim'l in which the chromic acid rinsing solution also contains phosphoric acid in an amount which is insufficient to lower the pH of the solution below 2.0.

4. The method of claim 1 in which the chromic acid rinsing solution is maintained at a temperature in the neighborhood of the boiling point.

ALFRED DOUI'Y. GERALD C. RONIIG.

Certificate of Correction Patent No. 2,403,426. July 2, 1946.

ALFRED DOUTY ET AL.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 1, first column, line 43, after the Word Water insert to make; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Oflice.

Signed and sealed this 3rd day of September, A. D. 1946.

LESLIE FRAZER,

First Assistant Oommz'aaz'mr of Patents. 

